A conventional gas turbine burns a hydrocarbon fuel, such as natural gas, by mixing the fuel with air, and uses the combustion gas to drive the turbine. The rotary motion of the turbine can then operate a generator, such as in an electric power plant.
The higher the temperature of the combustion gas entering the turbine, the higher the efficiency of the turbine. With present-day alloys in the turbine blades, this temperature is limited to 1200-1300.degree. C. However, absent perfect mixing of fuel and air, there will be islands of high fuel concentration where the temperature is high enough to form NOx. With improved mixing of fuel and air, the formation of NOx can be reduced to about 25 ppm with gas fuel and to about 40 ppm with liquid fuel. Catalytic combustion offers the possibility of reducing the NOx concentration to less than 10 ppm, which is already a requirement in some localities. With a catalytic combustor of the present invention, the temperature entering the turbine is the same as before, so that the efficiency of the turbine is not diminished. The benefit of the present invention is that the "window" of time and temperature which produces NOx is avoided, so that the output of NOx is below 10 ppm.
Temperature control in a catalytic combustor has been a serious problem. If the temperature in the combustor exceeds about 1000.degree. C., the catalyst will be destroyed. This problem can be at least partly solved by providing a combustor formed of strips, wherein the catalyst coating is provided on only one side of each strip. The latter structure is more fully described in U.S. Pat. No. 5,202,303, the disclosure of which is incorporated by reference herein.
It has been known to form a metal catalyst support by laying a corrugated strip of metal on a flat strip of metal, and winding the two strips upon themselves to make a spiral. It has also been known to fold a single strip back and forth upon itself to form the combustor. In the latter case, the positions of the folds can be chosen such that the combustor will have virtually any cross-sectional shape that is desired. More details about the latter techniques are provided in U.S. Pat. Nos. 4,402,871 and 4,576,800, the disclosures of which are incorporated by reference herein.
In a gas turbine, there is generally very little space within which to fit a catalytic combustor. Because of space limitations, it is desirable that the combustor be very thin. However, for a combustor formed of a single folded strip, or a combustor formed by winding a strip into a spiral, the folds or turns are likely to telescope outwardly, due to the force of the combustion gas, thereby destroying the combustor.
The present invention provides a catalytic combustor which solves the above-described problems, and which is practical for use in a gas turbine.